This study examines in newborn infants the relationships of plasma bacterial endotoxin activity with clinical parameters theoretically attributable to endotoxemia, and with alterations in selected serum complement components. Data accumulated thus far suggest the range of values for complement components Clq, C4, C3, and factor B found in our population of high risk neonates with a variety of illnesses. In some instances depletion of one or more of these components is evident, usually in association with severe illness, whether or not endotoxemia is present. In addition, certain serum complement component levels appear to be correlated with gestational or chronological age. Ongoing development of a radioimmunoassay for bacterial endotoxin has generated data on the cross-reactivity of antiserum to the deep rough J5 mutant of E. coil (Braude) with crude LPS containing extracts from a panel of blood and fecal isolates of Escherichia, various rough strains of Salmonella, and other organisms. A potentially useful new radioimmunoassay method for detecting antigens, involving labeled staphylococcal protein A, is also under active development.